This invention relates generally to blind fasteners and to a method of enabling setting of different size fasteners without necessity to change the wrench bits of the installation tool.
Blind fasteners commonly are used to join two workpieces together. One type of blind fastener generally comprises an internally threaded fastener body for insertion into aligned holes of the two workpieces, and an externally threaded cylindrical stem passing in threaded engagement through the fastener body. The inserted end of the stem has an enlarged stem head, and the outer end of the stem has a wrench engaging portion. Upon turning motion of the stem relative to the fastener body, the stem is moved in an axial outward direction through the fastener body. This axial outward movement causes a deformable sleeve around the stem and abutting against the stem head to deform around the fastener body to a fully set condition against the inner workpiece. The stem further may be provided with a localized weakened region or break groove adapted to shear the stem at a predetermined torque. The break groove preferably is located axially along the stem such that the stem shears in substantially flush relation to the outer portion of the fastener body, i.e., the fastener body head. The fastener body head normally is received in a countersunk, flush relationship to the outer workpiece, thus providing an aerodynamic surface after the fastener is set.
As the stem is turned through the fastener body, some means must be provided for preventing rotation of the fastener body inside the workpieces. One approach comprises providing a plurality of notches in the fastener body head for engagement by an installation tool having a non-rotating nosepiece fitting that is inserted into the notches. The fitting holds the fastener body against rotation, while a rotary wrench bit of the installation tool fits over the wrench engaging region and turns the stem through the fastener body. Another more recent approach, disclosed in copending application Ser. No. 583,029, involves mounting an internally threaded drive nut, such as a hex nut, on the stem in abutment with the outer surface of the fastener body head. The drive nut is held stationary by an appropriately configured fitting on the installation tool. The drive nut creates a frictional resistance for resisting turning slippage between abutting surfaces of the drive nut and the fastener body head during turning motion of the stem by the rotary wrench bit.
It is known that the size of the fastener used to connect the two workpieces varies depending upon several factors, such as the size and composition of the two workpieces. For example, heavy, thick workpieces generally require a larger size fastener than lightweight, thin workpieces. Therefore, as the size and composition of the workpieces varies, so does the size of the fastener. This includes variations in the dimensions of the fastener body, stem and drive nut. To install fasteners of different sizes, therefore, requires an installation tool having a different size nosepiece fitting and a different size rotary wrench bit for each size fastener. When various size fasteners are being installed in a particular work area, which is not uncommon when assembling structural members, for example, of an aircraft, the installer constantly may be required to use several installation tools, each one having a fitting and a wrench bit dimensioned for a particular size fastener. If access to several tools is not possible, the installer frequently may be required to change the fittings and wrench bits of the installation tool. This is time consuming and inconvenient.
If robotic installation of the fastener is contemplated, the time consuming problem of having to manually change components of the installation tool tends to undermine the reason for using robotic installation in the first place. Another related problem associated with robotic installation is that it may be difficult for the robotic installation tool to align with and engage the drive nut and wrench engaging region of the stem. This also is a consideration when using hand held installation tools. Most drive nuts are hexagonal in cross-section, while the wrench engaging region of most stems comprises wrenching flats. Depending upon the sophistication and mechanical capabilities of the robotic installation tool, the tool may not permit sufficient rotational adjustment of the nosepiece fitting and rotary wrench bit to align with and fit over the drive nut and wrenching flats, respectively. This poses additional time consuming problems and inconvenience.
Accordingly, there has existed a definite need for a blind fastener that despite its variation in size, may be installed by manual or robotic installation tools without having to change the wrench components of the tools each time a fastener of differing size is used. There further has existed a definite need for a method of installing blind fasteners of different sizes without necessity to change the wrench components of the installation tool. The present invention satisfies these needs and provides further related advantages.